Light emitting devices (LEDs), such as for example light emitting diodes, are used in displays for indoor and outdoor use. LED displays thereby aim at combining high light output and high resolution. Combining those properties requires a specific LED type approach. Different LED types exist. Surface mounted device-type (SMD-type) LEDs are LEDs that are mounted directly onto the surface of a printed circuit board. Lamp-type LEDs are LEDs whereby the light emitting device is mounted in a lamp housing.
For high resolution displays, often surface mounted devices (SMD) three-in-one LEDs are used. These LEDs combine three surface-mounted LED dies (RGB) in a polyamide body package. The LEDs are mounted on silverplated leadframes and the package is often closed at the front with an epoxy or silicone optical window. The SMD LEDs are becoming more efficient, but still provide only mediocre performance for outdoor use even when driven at high drive current. Often a white package is used to increase light output, but this yields lower contrast performance. The SMD type LEDs however have a small package size and can be very closely mounted, making it suitable for high resolution displays (3-10 mm pitch) and for delivering a good color compound and viewing angle performance. SMD-type LEDs are not suitable for exposure to environmentally harsh conditions like rain, frost and extreme heath. Leakage or deterioration due to environmental factors often occurs at edges where different materials touch each other, such as at a polyamide/silicone or polyamide/epoxy edge. Using an SMD LED as a light source in outdoor circumstances therefore is less appropriate due to its vulnerability to water-intrusion and failure due to water-leakage.
For high light output applications such as outdoor applications, in general, lamp-type LEDs are used. These LEDs consist of a single material body, normally epoxy material, and hold one LED die, be it red, green or blue, which is mounted on a silver plated lead frame and cup. These LEDs are well known for their efficiency and their resistance to aggressive environmental conditions. The lamp-type LEDs have the advantage of a wide viewing angle and a high brightness.
The lamp-type LEDs have the disadvantage that, since they are single color LEDs, they cannot be mounted very close together, yielding low color compound performance. Seen up close, which often happens e.g. with an indoor system, the LEDs will be seen as separated lightpoints and the colors will not blend together as one pixel.